Cathode heater element with a dark heat radiating coating and method of producing such

ABSTRACT

A cathode heater element is coated with a dark colored heat radiating porous layer of insulating material such as aluminum oxide and a refractory metal such as tungsten. The aluminum oxide is applied in an electrolytic bath, dried and impregnated with a tungstate solution. The layer is heated and sintered in a nitrogen hydrogen atmosphere. The heater element has improved heat transfer characteristics.

United States Patent Weiss 1 Aug. 26, 1975 [54] CATHODE HEATER ELEMENT WITH A 3,195,004 7/1965 Hassctt 313/345 X DARK HEAT RADIATING COATING AND 3,372,297 3/1968 Pearsall.... 313/337 1 3,401,297 9/1968 Feinleib 313/337 X METHOD OF PRODUCING SUCH 3,531,678 9/1970 Schiavone... 313/337 [75] inventor: Eberhard Weiss, Stuttgart, Germany 3,558,966 1/1971 Hill 313/345 3,737,714 6/1973 Theodosopoulos.... 313/337 1 Asslgneel lmernatlonal Standard Elecmc 3,765,939 10/1973 Reid 313/337 Corporation, New York, NY.

1 Filedl 1973 Primary ExaminerSaxfield Chatmon, Jr.

Appl. No.: 415,256

Foreign Application Priority Data Apr. 6, 1973 Germany .1 2317446 US. Cl. 313/345; 313/40; 313/340; 117/217 Int. Cl. H01J 1/14; H01J 19/10 Field of Search 313/337, 345, 340, 40, 313/41; 117/217 References Cited UNITED STATES PATENTS 4/1962 Etter 313/340 Attorney, Agent, or Firm-John T. OHalloran; Menotti J. Lombardi, Jr.; Edward Goldberg 5 7 ABSTRACT 1 Claim, 1 Drawing Figure CATHODE HEATER ELEMENT WITH A DARK HEAT RADIATING COATING AND METHOD OF PRODUCING SUCH BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heater element with a dark colored heat radiating coat of insulating material for indirectly heated cathodes of electron tubes and method of producing such a heater element.

2. Description of the Prior Art In the case of indirectly heated cathodes, the nature of the coat of insulating material on the conductive filament of the heater element is critical. The proper functioning of the entire assembly including the television picture tube, requires an adequate separation of potentials between the filament and the cathode tubing on which the emitting layer is positioned. For this purpose, the oxide of aluminum has proven to be useful as a good insulating material. As is well known, aluminum oxide is generally applied to the filament by electrophoretic deposition which forms a compact white layer.

In order to obtain a good transfer of heat from the heater element to the cathode tubing, and particularly to the emitting layer, efforts have been made to utilize not only the heat conduction but also the heat radiation effect. Heat radiation, however, is impeded by the white color of the oxide of aluminum. Therefore, attempts have been made for converting the white pure oxide of aluminum into a good heat radiator by the addition of dark coloring materials. As known in the prior art however, there are objections to such a dark coloring because previous experience has shown that the insulating property of the filament coating deteriorated to an undesirable degree.

For this reason, it was tried to provide the filament with a first white layer of aluminum oxide which was thereafter subjected to an externally applied dark coloring by a suitable method. These steps however did not provide any improvement with respect to the transfer of heat from the filament to the surface of the heater element, and especially not with respect to the heat radiation, and on the whole, only represent a compromise.

SUMMARY OF THE INVENTION It is therefore the object of the present invention to provide a heater element whose heat transfer properties are improved by utilizing internal heat radiation.

This object is attained by the use of a coating of insulating material applied to the filament which is completely porous and consists of a homogeneous dark colored sintered crystalline mixture of the insulating material, preferably aluminum oxide, A1 and of a refractory highmelting point metal such as tungsten. In accordance with many tests using conventional methods, the present heater element has proven to be completely satisfactory and has durable insulating properties. An important advantage of this heater element is in the fact that it is particularly suitable for use in instant heat type cathodes.

As regards the manufacture of the heater element, one method has proved to be particularly favorable. This is characterized by coating the filament with a layer of aluminum oxide and by simultaneously effecting electrolysis. The resulting porous insulating layer is then dried, the heater element is thereafter impregnated with an aqueous ammoniumtungstate solution, rinsed in a suitable liquid (such as methanol) and then dried again. Finally, the heater element is heated in a nitrogen and hydrogen atmosphere with the insulating layer being sintered. The invention will now be explained with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE schematically shows the construction of an indirectly heated cathode.

DESCRIPTION OF THE PREFERRED EMBODIMENT As show in the FIGURE, a cathode tubing 3 has a cap 4 welded at one end which carries the emitting layer 5. The structure is secured by the holder 6 which is mounted to a supporting disc 7. The heater element consisting of the filament 2 coated with the insulating layer 1 is inserted into the tubing 3.

From the point of view of thermal behavior, it is essential for the temperature difference between the filament 2 and the emitting layer 5 to be as small as possible, so that the operating temperature is reached as quickly as possible after current has been switched on. For this purpose the filament 2 is coated with a layer of insulating material 1 which is porous throughout, with no intermediate layers, and consists of a dark colored sintered crystalline mixture of the insulating material aluminum oxide, Al O and of a refractory highmelting point metal such as tungsten. The dark insulating layer 1, apart from the desired thermal properties, also provides very satisfactory results as regards insulating properties and long life, as well as only a very slight tendency toward melting during electrolysis. The layer is applied or deposited as follows.

The heater element is dipped or immersed into a suitable coating bath containing a mixture of aluminum oxide, water, alcohol and salt, and the mixture is dissociated upon application of an electric voltage. The aluminum oxide, which floats in the bath, is precipitated onto the cathode, i.e. on the filament. Simultaneously, during this electrophoretic process, hydrogen appears on the cathode due to the electrolysis. By suitably selecting the mixture ratio it is possible to achieve a particular porosity for the precipitating oxide of aluminum which also has a particular elasticity.

After-having reached the desired layer thickness, the coated filaments are removed from the coating bath, and dried. The dried heater elements are thereafter dipped into an aqueous ammonium-tungstate solution until the dried porous coat of insulating material is impregnated or saturated. In the course of a subsequent rising process, the heater element is again freed from a portion of the aqueous ammonium-tungstate solution. A suitable rinsing liquid may be methanol. This is followed by a further drying process.

After this the heater element is heated in an oven having a nitrogen/hydrogen atmosphere up to a temperature of about 1500 to 2000C at which the insulating layer is sintered.

What is claimed is:

l. A cathode heater element comprising a conductive filament having a coating of a dark colored heat radiating insulating material, said coating being porous throughout and including a non-emissive homogeneous dark colored sintered crystalline mixture of aluminum oxide and tungsten. 

1. A CATHODE HEATER ELEMENT COMPRISING A CONDUCTIVE FILAMENT HAVING A COATING OF A DARK COLORED HEAT RADIATING INSULATING MATERIAL, SAID COATING BEING POROUS THROUGHOUT AND INCLUDING A NON-EMISSIVE HOMOGENEOUS DARK COLORED SINTERED CRYSTALLINE MIXTURE OF ALUMINUM OXIDE AND TUNGSTEN. 